Let α and β be the roots of 3 x 2 + 4 x + 9 = 0 . Then ( 1 + α ) ( 1 + β ) can be expressed in the form b a , where a and b are coprime positive integers. Find a + b .
This problem is posed by Ron G.
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
We know that quadratic equations are of the form ax^2 + bx +c=0 where sum of roots = - ( \frac {b}{a} ) and product of roots = \frac {c}{a}. Here, sum of roots = (alpha + beta) = - ( \frac {4}{3} ) and product of roots = ( alpha * beta ) = \frac {9}{3} On expanding (1 + aplha)(1 + beta) we get 1 + ( alpha + beta) + (alpha * beta) . Putting values in this equation from above, we get 1 - ( \frac {4}{3} ) + ( \frac {9}{3} ) . Solve it to get /frac {8}{3} which is 11 in terms of a + b.
(1+α)(1+β)=1+(α+β)+αβ=1-4/3+9/3=8/3 a=8 , b=3 , a+b=3
We know that the sum of the roots is -4/3 and their product 3. So (1+a)(1+b)= 1+a*b +a+b =4-4/3=8/3.
Let S = α + β and P = α β
From Vieta's Formulae for sum and product of roots we obtain S = - 3 4 and P = 3 9 = 3
Let X = (1 + α ) (1 + β )
= 1 + α + β + α β
= 1 + S + P
= 1 - 3 4 + 3
= 3 8 = b a
Since a and b are co-prime positive integers, comparing from the final result we obtain a + b = 8 + 3 = 11 which is our answer.
Expanding ( 1 + α ) ( 1 + β ) , we get 1 + α + β + α β . In the given polynomial 3 x 2 + 4 x + 9 , a = 3 , b = 4 and c = 9 . According to Vieta's Formula, α + β = a − b = − 3 4 and α β = a c = 3 9 = 3 .
Hence, 1 + α + β + α β = 1 + ( 3 − 4 ) + 3 = 3 8 .
Since 8 and 3 are coprime, the answer is simply 8 + 3 = 1 1 .
(1+α)(1+β)=1+α+β+αβ By Viete's Theorem ,we have: α+β= -4/3 αβ =9/3 So (1+α)(1+β)=1+(-4/3)+9/3=8/3 a+b=8+3 =11
For any given quadratic equation a x 2 + b x + c , the roots are
α = 2 a − b + b 2 − 4 a c
β = 2 a − b − b 2 − 4 a c
Hence
α + β = 2 a − b + b 2 − 4 a c + 2 a − b − b 2 − 4 a c = 2 a − b + b 2 − 4 a c + ( − b ) − b 2 − 4 a c = 2 a − 2 b = − a b
α ⋅ β = 2 a − b + b 2 − 4 a c ⋅ 2 a − b − b 2 − 4 a c = 4 a 2 ( − b + b 2 − 4 a c ) ⋅ ( − b − b 2 − 4 a c ) = 4 a 2 ( − b ) 2 − ( b 2 − 4 a c ) 2 = 4 a 2 b 2 − b 2 + 4 a c = 4 a 2 4 a c = a c
In the question, the formula is 3 x 2 + 4 x + 9 .
Thus
( 1 + α ) ⋅ ( 1 + β ) = 1 + α + β + α ⋅ β = 1 − 3 4 + 3 9 = 3 8
8 and 3 are coprime, hence the answer is 8 + 3 = 1 1
Given: 3 x 2 + 4 x + 9 = 0
We have Sreedhara's formula, x = 2 a − b ± b 2 − 4 a c
i.e. x = 2 ∗ 3 − 4 ± 4 2 − 4 ∗ 3 ∗ 9 = 6 − 4 ± − 9 2 = 3 − 2 ± i 2 3
Now , let α = 3 − 2 + i 2 3 and β = 3 − 2 − i 2 3
Thus, ( 1 + α ) = 3 − 2 + i 2 3 + 1 = 3 1 + i 2 3
and ( 1 + β ) = 3 − 2 − i 2 3 + 1 = 3 1 − i 2 3
Therefore, ( 1 + α ) ( 1 + β ) = ( 3 1 + i 2 3 ) ( 3 1 − i 2 3 ) ( 1 + α ) ( 1 + β ) = ( 3 ∗ 3 1 2 − i 2 ( 2 3 ) 2 ) = 9 1 + 2 3 = 9 2 4 = 3 8 = b a
therefore , a = 8 and b=3 and
a+b = 8+3 = 11.
f ( x ) = 3 ( x − α ) ( x − β ) = 3 x 2 + 4 x + 9
⇒ f ( − 1 ) = 3 ( 1 + α ) ( 1 + β ) = 8 ⇒ ( 1 + α ) ( 1 + β ) = 3 8
⇒ a = 8 , b = 3 ⇒ a + b = 1 1
Problem Loading...
Note Loading...
Set Loading...
(1+α)(1+β)=1+(α+β)+αβ. According to Vieta's Formulas: If α and β are the roots of ax^2+bx+c=0,then α+β= - b/a and αβ= c/a. So α+β= - b/a = -4/3 and αβ= c/a = 9/3=3. Hence (1+α)(1+β)=1+(α+β)+αβ=1+(-4/3)+3=8/3. 8/3 is the form of a/b. Therefore a+b=8+3=11.